Directly connected you can have two pairs of 8 Ohm speakers.
If you get an impedance matching speaker controller you should be able to use all six and select between them.
Two Amps would allow you different inside and outside sound if that is of interest.
Yes, the goal would be able to use the two sets of speakers independently (e.g., one person watching tv inside on the 4 in-wall speakers, and another person listening to music on the two outside speakers).
Is there anyway to manage this without spending another $700 on a Sonos Amp?
The Amp can only provide one source/output so no.
For outdoor use have you looked at the Sonos portables, less than an amp.
Move and Roam:
https://www.sonos.com/en-us/shop/portable-set-move-2-roam-2-black
Thank you. The previous owner had installed Yamaha speakers outside so will need something to power those.
Maybe a used but recent (2016?) Connect Amp that runs S2 firmware?
Got it, thank you so much Stanley!